The BALB/3T3 clone A41-1-1 cell line was studied with the following assays: cytotoxicity, ouabain resistance (oua-r) mutations, morphological neoplastic transformation, DNA damage and repair as measured by alkaline elution and removal of alkylated DNA adducts as measured by HPLC. The relative levels of response for these biological end points were determined after treatment with the alkylating agents, N-methyl-N'-nitro-N- nitrosoguanidine (MNNG) and N-ethlnitrosourea (ENU). A few dissociation phenomenon, reported last year, was confirmed and further investigated this year. Duration of the exposure to a medium containing a given initial concentration of ENU determined maximal responses that differed remarkably for different end-points. Short exposure duration times (5 min) were sufficient to induce maximal levels of oua-r mutations and of DNA single strand breaks (ssb), whereas the maximal induction of transformation as well as of cytotoxicity required 45-60 mins. Additional studies of exposure time-dependent responses were conducted with ENU in Chinese hamster ovary (CHO) cells, for which both oua-r mutations and 6-thioguanine resistance (6-TG-r) mutations can be assayed. The induction kenetics of oua-r mutations and ssb in CHO cells showed early maximal induction analogous to that observed in the BALB/3T3 cells, whereas 6-TG-r mutations were induced at progressively higher rates in time, consistent with a theoretical curve based on a linear relationship to expotential decay of ENU in the medium. BALB/3T3 cells removed 06-ethylguanine rapidly and N3-ethyladeine to a lesser degree, whereas no removal within 60 min was detected for N7-ethylguanine, 04-ethylthymine and ethyl-phosphotriesters. The CHO cells, however, were not capable of repairing 06-ethylguanine lesions. Repair of 06-alkylguanine, therefore, is unlikely to explain the early maximal response of oua-r mutations and ssb. The temporal dissociation model is being further studied to clarify the basis for the long induction times for transformation.